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SAIT System Configuration - Model JDR-500-TL-UHV-STM

Following the milestone “A 10 mK Scanning Probe Microscopy Facility” that Janis developed for the National Institute of Standards and Technology (NIST) [REVIEW OF SCIENTIFIC INSTRUMENTS, 81, 121101 (2010)], Janis has successfully developed another top-loading mK STM dilution refrigerator (DR) for Samsung Advanced Institute of Technology (SAIT) in Korea with several enhanced design and performance features as compared with the earlier system. The SAIT system includes a 15.5 Tesla superconducting magnet with compensation coil and has a 1.5” diameter central access into the UHV space. Using a top manipulator, the large bore central access allows an STM with in-situ prepared sample and probe tip to be translated into the magnet center without warming up the system above 4 K.

Other novel features incorporated in the SAIT system include:

Comprehensive UHV compatible wires and coaxial cables are installed to fit the user’s special applications.

User-friendly mechanical heat switch is installed for fast pre-cooling of the DR stage.

The SAIT system has been successfully installed on site and reached the following performance specifications:

The mixing chamber has reached a base temperature of 6.6 mK per the CMN sensor calibrated with the fixed superconducting point device (FPD).

More than 300 µW @ 100 mK cooling power has been achieved.

The solenoid has reached maximum magnetic field of 15.5 T @ 4.2K with DR at base temperature.

To prove ULT STM performance and atomic resolution, the user has been studying graphite and graphene only so far. A “standard” superconducting sample will be used to measure the BCS gap and determine the electronic temperature.

The user has disclosed, “We now have a UHV ultralow temperature scanning probe microscope working with a base temperature of 6.6 mK and the maximum magnetic field of 15.5 T.”

NIST System Configuration - Model JDR-500-UHV for STM Application

A custom UHV compatible JDR-500 dilution refrigerator (DR) was tested at Janis Research Company in preparation for installation at NIST’s new Center for Nanoscale Science and Technology (CNST). The system includes a 15 Tesla superconducting magnet with a compensation coil and has central access into the UHV space. Using a top manipulator, the large bore central access allows an STM with in-situ prepared sample and probe tip to be translated into the magnet center without warming up the system above 4 K. An integral pre-cooling circuit is used to cool the system to 4 K without the need for any helium exchange gas.

The DR itself has been modified to meet user specifications (see photos below). First of all there is a 1.5” central access hole passes through every plate and chamber, including the Mixing Chamber (MC). There are two additional 0.95” side access holes above the MC. The DR also includes additional smaller ports for use in support of auxiliary needs such as shutter manipulation.

The gas handling system includes a bellows pump/compressor in addition to Alcatel Roots and mechanical vane pumps. The mixture Condensing Unit (MCU) contains a Joule-Thomson (JT) stage as well as a 1 K pot with computer-controlled and manual needle valves. The two condensation stages can be used interchangeably, with the JT stage utilized for low vibrational measurements. Initial mixture condensing is done in 1 K pot mode, while normal circulation is primarily in JT mode. The Janis manual Gas Handling System includes digital gauges for future computer control of the DR operation. UHV compatible RuO2 thermometers were provided by the NIST group (six in total installed), while Janis supplied UHV versions of a CMN thermometer and a Fixed Point Device. After calibrating the CMN thermometer against superconducting fixed points, a laboratory scale was created for this thermometer and was then used to calibrate RuO2 thermometers that were installed at the MC.

The UHV compatible DR insert employs all metal sealed Conflat and VCR flanges (no indium sealed flanges were used). Non-magnetic austenitic stainless steel and copper alloys are used below the IVC flange. All surfaces are treated for low out-gassing, including electro-polished stainless steel and 24 carat gold plated OFHC copper. Structurally the DR stage was reinforced to have high intrinsic resonance frequencies and high axial and torsional strength. Special UHV compatible shielded wires were developed for this application and are now becoming standard in Janis UHV compatible systems. The Insert can be baked at 100 °C in a special baking chamber while the DR unit is thermally controlled using a dedicated pre-cooling line. The same line is used for initial pre-cooling of the DR unit, eliminating the need for exchange gas. A super-efficient vapor shielded liquid helium dewar with a total evaporation rate (including magnet support and DR) of less than 0.7 L/h guarantees a long time (9 – 10 days) between liquid helium transfers.

The first test of the system confirmed that it achieves a 10 mK base temperature with approximately 100 shielded wires attached to the MC, and provides a cooling power of greater than 300 µW at 100 mK in both the 1 K Pot and JT operating modes. No heating was observed at the CMN thermometer at 10 mK while sweeping magnetic fields up to 10 Tesla at moderate rates.